An innerspring assembly such as used for the manufacture of innerspring mattresses comprises spaced, parallel, substantially rectangular border wires of relatively stiff wire between which are assembled rows of coils with the top and bottom ends situated in the planes of the border wires. Spiral springs are employed to connect the top and bottom ends of the coils to the border wires and to each other. Thus, the border wires are unsupported relative to each other, except by the intervening coils. Consequently, if pressure is brought to bear near the edges or corners, the top of the structure will yield downwardly relative to the bottom and this is especially pronounced at the corners. Because of the closer spacing of coils along the sides, and ends, there is less yield at the sides and ends. Reinforcing spring members have been placed between the border wires along the sides and ends to prevent excessive yield and sometimes to enable providing the desired resistance to yield with lighter gauge wire. It is the purpose of this invention to provide supplemental spring means at the corners which has not been done heretofore to overcome the softness at the corners and further to provide means for this purpose which may be built into the assembly at the time of manufacture of the assembly, or may be supplied to a spring manufacture as a unit which he may then assemble to an already manufactured spring assembly either to improve its resistance to yield or to enable using a lighter gauge wire for the interior of the assembly without collapse at the corners.